This invention relates to gamma-titanium aluminide alloys.
Titanium alloys have found wide use in gas turbines in recent years because of their combination of high strength and low density, but generally, their use has been limited to below 600.degree. C. due to inadequate strength and oxidation properties. At higher temperatures, relatively dense iron, nickel, and cobalt base super-alloys have been used. However, lightweight alloys are still most desirable, as they inherently reduce stresses when used in rotating components.
While major work has been performed since the 1950's on lightweight titanium alloys for higher temperature use, none has proved suitable for engineering application. To be useful at higher temperature, titanium alloys need the proper combination of properties. In this combination are properties such as high ductility, tensile strength, fracture toughness, elastic modulus, resistance to creep, fatigue and oxidation, and low density. Unless the material has the proper combination, it will not perform satisfactorily, and thereby be use-limited. Furthermore, the alloys must be metallurgically stable in use and be amenable to fabrication, as by casting and forging. Basically, useful high temperature titanium alloys must at least outperform those metals they are to replace in some respect, and equal them in all other respects. This criterion imposes many restraints and alloy improvements of the prior art once thought to be useful are, on closer examination, found not to be so. Typical nickel base alloys which might be replaced by a titanium alloy are INCO 718 or IN100.
Heretofore, a favored combination of elements with potential for higher temperature use has been titanium with aluminum, in particular alloys derived from the intermetallic compounds or ordered alloys Ti.sub.3 Al (alpha-2) and TiAl (gamma). Laboratory work in the 1950's indicated these titanium aluminide alloys had the potential for high temperature use to about 1000.degree. C. But subsequent engineering experience with such alloys was that, while they had the requisite high temperature strength, they had little or no ductility at room and moderate temperatures, i.e., from 20.degree. to 550.degree. C. Materials which are too brittle cannot be readily fabricated, nor can they withstand infrequent but inevitable minor service damage without cracking and subsequent failure. They are not useful engineering materials to replace other base alloys.
The two titanium aluminides, Ti.sub.3 Al and TiAl, could serve as a base for new high temperature alloys. Those skilled in the art recognize that there is a substantial difference between the two ordered titanium-aluminum intermetallic compounds. Alloying and transformational behavior of Ti.sub.3 Al resemble those of titanium as they have very similar hexagonal crystal structures. However, the compound TiAl has a face-centered tetragonal arrangement of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in the earlier literature. Therefore, the discussion hereafter is largely restricted to that pertinent to the invention, which is within the TiAl gamma phase realm, i.e., about 50Ti-50Al atomically and about 65Ti-35Al by weight.
The effect of hydrogen on the physical and mechanical properties in alpha, beta and alpha-beta titanium alloys, i.e., titanium-aluminum alloys containing up to about 14 atomic percent (8 wt %) aluminum, has received considerable attention. It has been used to embrittle titanium to facilitate its comminution by mechanical means to form titanium metal powders. In such techniques hydrogen is diffused into the titanium at elevated temperatures, the metal is cooled and brittle titanium hydride is formed. The brittle material is then fractured to form a powder. The powder may then have the hydrogen removed or a compact may be formed of the hydrided material which is then dehydrided.
Hydrogen has the effect of increasing the high temperature ductility of titanium alloys. This characteristic has been used to facilitate the hot working of titanium alloys. Hydrogen is introduced to the alloy which is then subjected to high temperature forming techniques, such as forging or superplastic forming. The presence of hydrogen allows significantly more deformation of the metal without cracking or other detrimental effects, Lederich et al, U.S. Pat. No. 4,415,375.
Hydrogen has also been used as a temporary alloying element in an attempt to alter the microstructure and properties of titanium alloys. In such applications, hydrogen is diffused into the titanium alloys, the alloys heat treated by various means including cooling to room temperature and then heated to remove the hydrogen. Vogt et al, U.S. Pat. No. 4,680,063. Alternatively, hydrogen is diffused into the titanium alloys and then removed from the alloys. Smickley et al, U.S. Pat. No. 4,505,764.
In the as-processed condition, cast TiAl has a large average grain size, with grain size ranging from about 100 microns to 1000 microns, or greater. As discussed above, hydrogen has been employed very effectively to refine the microstructure of conventional Ti alloys, i.e., Ti alloys containing up to about 8 wt % Al. Unfortunately, the addition of hydrogen to gamma-titanium aluminide is not possible conventionally because of the very low solubility of hydrogen in the face-centered tetragonal matrix. What is desired is a method for adding hydrogen to the gamma-titanium aluminide which will allow enhanced processability and/or subsequent refinement of the microstructure of gamma-titanium aluminide in a manner similar to that possible in conventional titanium alloys and the intermetallic compound Ti.sub.3 Al.
Accordingly, it is an object of the present invention to provide a method for adding hydrogen to titanium aluminide (TiAl) to allow enhanced processability and microstructural refinement.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.